The present invention relate.s to an electrode for nonaqueous electrolyte battery.
In recent years, with the development of electronic apparatus, the appearance of a novel high performance battery has been expected more and more. At present, as primary battery used as power supply for electronic apparatus there is mainly used manganese dioxide-zinc battery. As secondary battery used as power supply for electronic apparatus there is mainly used nickel battery such s nickel-cadmium battery, nickel-zinc battery and nickel-metal hydride battery or lead-acid battery.
As the electrolyte solution for these batteries there is used an aqueous solution of alkali such as potassium hydroxide or an aqueous solution of sulfuric acid or the like. The theoretical decomposition voltage of water is 1.23 V. A battery system having an electromotive force of higher than this value is liable to experience decomposition of water that makes it difficult to stably store an electric energy. Therefore, nothing but a battery system having an electromotive force of about 2 V at highest has been put into practical use. Accordingly, as the electrolyte solution for high voltage batteries having an electromotive force of not lower than 3 V there must be used a nonaqueous electrolyte solution. A typical example of such a battery is a so-called lithium battery comprising lithium as a negative electrode.
Examples of primary lithium battery include manganese dioxide-lithium battery, and carbon fluoride-lithium battery. Examples of secondary lithium battery include manganese dioxide-lithium battery, and vanadium oxide-lithium battery.
A secondary battery comprising metallic lithium as a negative electrode is disadvantageous in that it is liable to internal short due to dendritic growth of metallic lithium and thus has a reduced life. Further, since metallic lithium has a high reactivity, such a secondary battery can be hardly provided with a high safety. In order to eliminate these difficulties, a so-called lithium ion battery comprising graphite or carbon instead of metallic lithium and lithium cobaltate or lithium nickelate as e positive electrode has been devised and used as a high energy density battery. In recent years, with the expansion of usage, batteries having higher performance and safety have been desired.
Unlike lead-acid battery, nickel-cadmium battery and nickel-metal hydride battery, which comprise an aqueous solution as an electrolyte, lithium battery and lithium ion battery (hereinafter collectively referred to as xe2x80x9clithium-based batteryxe2x80x9d) comprise a combustible organic electrolyte solution as an electrolyte. For the sake of safety, the lithium-based battery needs to be equipped with various safety elements such as safety valve, protective circuit and PTC element which add to cost. The conventional batteries comprising a nonaqueous electrolyte solution are liable to experience vaporization of the electrolyte solution due to heat generation during shortcircuiting or other troubles, which causes a sudden rise in the inner pressure thereof. In a lithium ion battery comprising a carbon-based negative electrode represented by LixC6, as the percent utilization of negative electrode is raised and intercalation proceeds, the rise in the temperature and pressure in the battery due to internal short or other troubles causes lithium and carbon in the negative electrode to react with each other to produce lithium carbide with the generation of heat. As a result, the inner pressure of the battery shows a sudden further rise. Thus, this lithium ion battery leaves something to be desired in safety. Accordingly, at present, the percent utilization of the carbon-based negative electrode is limited to less than 60% (LixC6, 0xe2x89xa6xc3x97 less than 0.6) taking into account the safety, making it impossible to obtain a practical battery having a high energy density.
Further, since the lithium-based battery exhibits a higher battery voltage than the aqueous solution battery, the electrolyte solution undergoes decomposition by oxidation or reduction while it is kept being charged. Thus, the lithium-based battery has deteriorated charged storage properties.
In an attempt to improve the safety and charged storage properties of battery, the use of a solid electrolyte having a reduced chemical reactivity instead of electrolyte solution has been proposed (Electrochimica Acta 40 (1995) 2119). Further, in an attempt to render the battery shape flexible, simplify the production process and reduce the production cost, the application of a solid polymer electrolyte has been proposed.
Concerning ionically-conductive polymers, many complexes of polyether such as polyethylene oxide and polypropylene oxide with alkali metal salt have been studied. However, polyether cannot provide a high ion conductivity while maintaining a sufficient mechanical strength. Further, the ion conductivity of the polyether is drastically affected by temperature and thus cannot provide a sufficient ion conductivity at room temperature. Thus, the use of comb-shaped polymer having polyether in its side chains, copolymer of polyether chain with other monomers, polycyloxane having polyether in its side chains or crosslinked polyphosphazene or polyether has been attempted.
In an ionically-conductive polymer having a salt dissolved therein such as polyether-based polymer electrolyte, both cation and anion migrate. Such an ionically-conductive polymer normally exhibits a cation transport number of not more than 0.5 at room temperature. Thus, in an attempt to provide a lithium ion transport number of 1, ionically-conductive polymer containing an anionic group such as xe2x80x94SO3xe2x88x92 and xe2x80x94COOxe2x80x94 has been synthesized. However, lithium ion is strongly constrained by anionic group in such a compound. Thus, such a compound cannot be hardly used in lithium-based batteries.
Further, the application of a gel solid electrolyte prepared by impregnating a polymer with an electrolyte solution to lithium-based batteries has been attempted. Examples of polymer used in the gel solid electrolyte include polyacrylonitrile (J. Electrochem. Soc. 137 (1990) 1657, J. Appl. Electrochem. 24 (1994) 298), polyvinylidene fluoride (Electrochimica Acta 28 (1983) 833, 28 (1983) 591), polyvinyl chloride (J. Electrochem. Soc. 140 (1993) 196), polyvinylsulfone (Electrochimica Acta 40 (1995) 2289, Solid State Ionics 70/71 (1994) 20), and polyvinylpyrrolidinone. In an attempt to reduce the degree of crystallization of polymer, facilitating the impregnation thereof with an electrolyte solution and hence improving the ion conductivity, the use of a copolymer of vinylidene fluoride with hexafluoropropylene has been proposed (U.S. Pat. No. 5,296,318). The preparation of a lithium ionically-conductive polymer film which comprises drying a latex such as nitrile rubber, styrene butadiene rubber, polybutadiene and polyvinyl pyrrolidone to prepare a polymer film, and then impregnating the polymer film with an electrolyte solution has been proposed (J. Electrochem. Soc. 141 (1994) 1989, J. Polym. Sci. A 32 (1994) 779). Referring to the preparation of a polymer electrolyte from a latex, the mixing of two kinds of polymers has been proposed to provide a mixture of a polymer phase which can hardly be impregnated with an electrolyte solution and thus maintains a high mechanical strength and a polymer phase which can easily be impregnated with an electrolyte solution and thus exhibits a high ion conductivity and hence provide a polymer film which gives a high mechanical strength and a high ion conductivity.
Further, a solid electrolyte obtained by filling the pores in a microporous polyolefin film with a polymer electrolyte for the purpose of increasing the mechanical strength and improving the handleability of polymer electrolyte film (J. Elecrochem. Soc. 142 (1995) 683) and a polymer electrolyte comprising an inorganic solid electrolyte powder incorporated therein for the purpose of improving the ion conductivity and increasing the cation transport number (J. Power Sources 52 (1994) 261, Electrochimica Acta 40 (1995) 2101, 40 (1995) 2197) have been reported.
As mentioned above, various polymer electrolytes have been proposed. However, no polymer electrolytes giving essential solution to the problem of diffusion of lithium ion have been reported. Thus, the properties of nonaqueous batteries have not been sufficient as compared with that of aqueous batteries. In a lithium-based battery, the majority of lithium ions taking part in the electrode reaction during charge-discharge reaction is not dissolved in the electrolyte originally, but released from the active material in an opposing electrode. Thus, the moving distance of lithium ion is long. Further, the transport number of lithium ion in the electrolyte of a lithium-based battery at room temperature is normally not more than 0.5 while the transport number of proton and hydroxide ion in the aqueous battery is close to 1. In a lithium-based battery, the moving rate of ion in the electrolyte is governed by the diffusion of ion. Moreover, since an organic electrolyte has a higher viscosity than an aqueous solution, it allows ion diffusion at a lower rate than an aqueous solution. Accordingly, a lithium-based battery comprising an organic electrolyte solution is disadvantageous in that it is inferior to an aqueous battery in high rate charge-discharge properties. This problem becomes very remarkable at low temperatures. A lithium-based battery comprising a polymer electrolyte solution allows ion diffusion at a even lower rate than one comprising an organic electrolyte instead of electrolyte solution and thus is disadvantageous in that it is more inferior to a nonaqueous battery comprising an organic electrolyte solution in high rate charge-discharge properties.
As the solid electrolyte to be used in a lithium-based battery there has been applied a lithium ionically-conductive inorganic solid electrolyte besides polymer electrolyte. However, such a lithium ionically-conductive inorganic solid electrolyte has some disadvantages. In other words, it cannot provide a sufficient lithium ion conductivity. Further, it lacks resistance to reduction-oxidation. Moreover, the volumetric expansion and shrinkage of the active material during charge-discharge causes the active material to be peeled off the inorganic solid electrolyte. Thus, such a lithium ionically-conductive inorganic solid electrolyte has never been put into practical use.
Since an organic electrolyte is combustible, a lithium-based battery comprising an organic electrolyte needs to be equipped with various safety elements such as safety valve, protective circuit and PTC element for the sake of safety, adding to cost. Further, since a lithium-based battery exhibits a higher battery voltage than an aqueous solution battery, the electrolyte solution undergoes decomposition by oxidation or reduction while it is kept being charged. Thus, a lithium-based battery has deteriorated charged storage properties. A lithium-based battery comprising a polymer electrolyte instead of an electrolyte solution for the purpose of improving the safety and charged storage properties thereof is disadvantageous in that it allows ion diffusion in the electrolyte at a reduced rate and thus cannot perform charging and discharging at a high rate. Such a lithium-based battery exhibits remarkably deteriorated high rate charge-discharge properties at low temperatures. The present invention has been worked out in the light of these problems. The present invention provides an electrode for nonaqueous electrolyte battery which exhibits excellent safety and charged storage properties and good high rate charge-discharge properties.
One aspect of the present invention concerns an electrode for nonaqueous electrolyte battery based on quite a novel principle comprising a particulate active material having a porous film formed thereon. Thus, a nonaqueous electrolyte battery excellent in safety, charged storage properties and high rate charge-discharge properties can be provided.
Another aspect of the present invention concerns an electrode for nonaqueous electrolyte battery based on quite a novel principle comprising an active material having a filler held in pores. Thus, a nonaqueous electrolyte battery excellent in safety, charged storage properties and high rate charge-discharge properties can be provided.
A further aspect of the present invention concerns an electrode for nonaqueous electrolyte battery based on quite a novel principle comprising an active material which undergoes volumetric expansion and shrinkage during charging and discharging, having a filler held in pores. Thus, a nonaqueous electrolyte battery excellent in safety, charged storage properties and high rate charge-discharge properties can be provided.
A still further aspect of the present invention concerns an electrode for nonaqueous electrolyte battery based on quite a novel principle comprising a particulate active material having a porous ionically-conductive film formed thereon. Thus, a nonaqueous electrolyte battery excellent in safety, charged storage properties and high rate charge-discharge properties can be provided.